1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing a solid fuel with low-rank coal
2. Description of the Related Art
A general process for manufacturing a solid fuel with low-rank coal is disclosed in Japanese Unexamined Patent Application Publication No. 7-233383. The process disclosed in the Patent Publication includes the following procedure: Porous coal is mixed with a mixed oil containing a heavy oil and a solvent oil to produce a slurry. Heating the resulting slurry dehydrates the porous coal, and fills the pores of the porous coal with the mixed oil. Next, the slurry is subjected to a solid-liquid separation. Low-rank coal is used as the porous coal.
A solid fuel that can be safely transported and stored due to its low spontaneous combustibility and that also has a high calorific value can be manufactured by the process including dehydration disclosed in the above-described Patent Publication.
Since porous coal (low-rank coal) has a high moisture content, the porous coal causes an increase in transport cost, i.e., a decrease in transport efficiency in proportion to the moisture content, and has a low calorific value. Thus, porous coal is preferably dehydrated. However, when porous coal is dehydrated by a general drying process, there is a danger of a spontaneous combustion accident caused by oxygen adsorption on active sites, which are present in the pores of the dehydrated porous coal, followed by oxidation.
In the above-described process for manufacturing a solid fuel, moisture in the pores is evaporated by heating an untreated slurry composed of porous coal and a mixed oil containing a heavy oil and a solvent oil. At the same time, the inner surfaces of the pores are covered with the mixed oil. Finally, the pores are filled with the mixed oil, in particular, with the heavy oil dominantly. Therefore, the adsorption of oxygen to such active sites in the pores and oxidation are suppressed, thus blocking spontaneous combustion. Furthermore, the resulting solid fuel has a high calorific value because the porous coal is dehydrated and the pores are filled with the mixed oil by heating. In this way, a solid fuel that can be safely transported and stored due to its low spontaneous combustibility and that also has a high calorific value can be manufactured by the process including dehydration.
In the process for manufacturing a solid fuel disclosed in the above-described Patent Publication, porous coal (low-rank coal) pulverized with a pulverizer is used as a material for the untreated slurry. That is, the untreated slurry is produced by mixing the porous coal pulverized with a pulverizer and a mixed oil containing a heavy oil and a solvent oil in a mixing tank.
By heating the untreated slurry, the porous coal is dehydrated, and the pores of the porous coal are filled with the mixed oil. This dehydration by heating is generally performed in an evaporator. In the mixing tank and the evaporator, the slurry state is maintained by circulating the slurry with a slurry pump while stirring, thus preventing the deposition of the coal caused by sedimentation.
After the porous coal is dehydrated and the pores of the porous coal are filled with the mixed oil by heating, the resulting heated slurry (hereinafter, referred to as “dehydrated slurry”) is subjected to solid-liquid separation to separate the dehydrated slurry into a solid and a liquid. The resulting solid is composed of porous coal containing the mixed oil in its pores and can be used as a powdered solid fuel. Briquetting the resulting solid produces a briquette solid fuel. The separated liquid that is mainly composed of oil can be recycled as oil used to produce an untreated slurry. That is, the separated liquid that is used as a recycling oil is returned to a step of producing an untreated slurry.
The solid-liquid separation of the dehydrated slurry is performed with, for example, a centrifuge. The separated liquid contains finely powdered coal that was not completely separated from the dehydrated slurry. For example, when the dehydrated slurry is subjected to solid-liquid separation with a decanter centrifuge, fine particles (finely powdered coal) having a diameter of about 50 μm or less are not separated and remain in the separated liquid.
Such a separated liquid is returned to a step of producing an untreated slurry. Hence, the content of finely powdered coal in a recycling oil increases in every cycle, thus reducing the flowability of the untreated slurry. Therefore, the process does not smoothly proceed. If the untreated slurry does not flow, the process stops.
When the solid-liquid separation of the dehydrated slurry is performed with, for example, a distillation unit having an excellent ability for the solid-liquid separation, a separated liquid hardly contains finely powdered coal. The use of the resulting separated liquid as a recycling oil can prevent the reduction in the flowability of the untreated slurry caused by the contamination of finely powdered coal. However, such a unit has disadvantages of the long time required for the separation and being noneconomical due to high operational costs compared with a centrifuge. Therefore, such a unit is difficult to be applied.